The QCM (Quartz Crystal Microbalance) method that utilizes the resonance phenomenon of a crystal oscillator allows for the detection of even very small mass changes using a simple apparatus, not only for measurements in a gaseous phase but also in a liquid phase. For this reason, the QCM method has been widely used for, for example, gas sensors, film thickness sensors, chemical sensors, and biosensors that measure interactions of biological materials such as DNA and protein.
Conventionally, the QCM method is applied in a variety of measurements by being used to measure the resonant frequency that results from oscillation, or the resonant frequency obtained by sweeping a frequency using a device such as an impedance analyzer and a network analyzer.
In QCM measurement performed in a gaseous phase, only the mass of the material adhered to an electrode surface is detected as an amount of frequency change, and accordingly conversion into density is relatively easy.
However, in the measurement of solution density and viscosity, it has been difficult to separately measure the density and the viscosity of the solution using the conventional QCM method, because, unlike measurement in a gaseous phase, the frequency change due to the solution resistance on the oscillating surface of a crystal oscillator has a correlation with the product of density and viscosity.
Thus, if the density and viscosity were both unknown, the values of density and viscosity cannot be determined using the conventional QCM method.
As a countermeasure, U.S. Pat. No. 5,741,961 and U.S. Pat. No. 5,798,452 propose methods for measuring solution density and viscosity using a sensor provided with two or more detectors. At least one of the detectors is used as a reference detector that measures the frequency due to solution resistance, and the solution resistance component of the reference detectors is removed from the measurement results of the other detectors so as to measure the density and viscosity of the solution.
However, it is laborious to provide a plurality of detectors for a single piezoelectric plate, or to use a plurality of sensors. In addition to being laborious, it increases the manufacturing cost of the sensor. Another problem is the individual differences among the detectors.
Further, drawing a standard curve with a single sensor requires preparation of a plurality of standard samples, and it takes time to start the actual measurement. Further, because the density and viscosity can only be determined from the resonant frequency that contains the information of both density and viscosity, measurement accuracy is considerably low.